Mechanism of the light state transition in photosynthesis. III. Kinetics of the state transition in Porphyridium cruentum

Abstract

The kinetics of the light state transition in the red alga Porphyridium cruentum were studied in low intensities of initiating light and in saturating flashes brief enough to elicit single turnovers of the photochemical apparatus. We confirm that the state transition is dose-dependent, but also found that the transition to state 2 was biphasic. The slow phase was correlated with the induction of photosynthesis and was eliminated if the preceding time spent in state 1 was very short. The full transition to state 1 developed following a minimum of 15 turnovers of Photosystem I, and the optimal frequency for the flash sequence was determined to be 2.5 Hz. In contrast, the turnover time required for the transition to state 2 was found to be smaller than 30 ms. The data are consistent with a mechanism we have recently proposed for the state transition in organisms that contain phycobilisomes. The mechanism proposed involves a small conformational change within the thylakoid that is brought about by localized differences in electrochemical potential. A Photosystem-I-generated potential difference of H + is prerequisite for the initiation of state 1 and, under certain conditions, a localized electric-field generated by Photosystem II may play a significant role in the transition to state 2.